Fixation systems for bones with a load carrier and bone screws which can be inserted and fixed in the load carrier, e.g. plate, nail and fixator systems, are used in order to surgically connect bone fragments with one another.
Bone screw and bone plate can in this case also be interlocked with one another at a fixed angle. This new generation of implants, having a multi-directionally fixed-angle connection between the screw head and the bone plate hole, are distinguished by a higher capability with unchanging or reduced material expenditure.
Investigations have shown that the load flow in these fixed-angle systems is characterized in that the loads are not transferred uniformly, via the screws, from a bone fragment to the bone plate and, via the plate and the screws, to the other bone fragment, but that the first screw adjacent to the fracture and the plate components surrounding the plate hole transfer approximately 50-60% of the overall forces and loads, the following, that is to say second, screw and the plate components surrounding the plate hole transfer 20-30%, and the next, that is to say third, screw and the plate components surrounding the plate hole transfer approximately 10%.
The reason why this fact is of great importance is because, when assuming the previously used plate design, there is a threat of failure of the system in the region of the first plate hole adjacent to the fracture, to be precise in such a fashion that either the neck region of the screw or the plate, level with this first hole, deforms or breaks as a result of overloading or fatigue.
This problem is solved in European Patent 1 211 994 B1 by virtue of the fact that the load carrier is strengthened by widening or has a region with stronger material. Said material is strongest in the region of the first hole for the bone screw, located proximally to the fracture or instability zone; it is less strong in the region of the second, distally subsequent hole; and weakest in the region of the third, distally subsequent hole.
Furthermore, it is possible to react by simultaneously increasing the thickness of the screw core, and hence the screw, so that the load stability of the respective plate hole/screw unit constitutes an addition of the strength of the plate and the strength of the screw.
The disadvantage lies in larger dimensions and thus an increased cover of the bone surface, and increased lifting of the soft parts which come to rest above the plate. This is the equivalent of increasing the surgical wound and hence also of increasing the complexity of the surgical procedure. In certain areas of application, in which the size of the implants is limited by the anatomy, e.g. in the region of the hand, the fixation systems disclosed in EP 1 211 994 B1 can hardly be used, not least because of the additional widening or increased thickness of the load carrier.